The past several decades have witnessed the development of ever smaller electrical circuit components at the chip level. In order to take fullest advantage of achievements in electrical circuit miniaturization; however, one must package the resultant printed circuit cards containing these chips in an efficient manner. Clearly, the packaging of printed circuit cards in tight spaces is a direct logical extension of increasing chip level circuit densities. It should also be noted that the tight packaging of integrated circuit chips on printed circuit cards and the correspondingly dense packaging of the printed circuit cards is a design goal that is carried out for more than just the convenience of compactness. Compactness provides shorter distances between circuit components which, in turn, serve the very desirable goal of being able to operate the circuits effectively at higher frequencies, thus increasing the speed of numerous different forms of electrical systems, including but not limited to data processing systems.
Moreover, mainly for reasons associated with long-term system operation and reliability, it is likewise very desirable to be able to easily insert and remove these printed circuit cards even when they are disposed in very tight spaces. The insertion and removal operations are also provided as an important part of a “hot-pluggability” function which is very desirable for “on the fly” repairs, replacements, maintenance and upgrades. Accordingly, to whatever extent possible, packaging designs should be: economical to produce; function smoothly, require little or no maintenance; be producible from inexpensive, readily available materials; and be reliably operable over a large number of insertion and removal operation cycles.
Yet one other concern arises in electrical systems as circuit feature size shrinks, operating frequencies increase and packaging densities grow larger, namely, the generation of electromagnetic interference (EMI). Electronic circuit packaging designs should thus also be compatible with structures and configurations that are employed to prevent the leakage of electromagnetic interference.
Packaging designs should also include structures, which actually contribute positively to the containment of electromagnetic interference. There is an ever-increasing problem of electromagnetic interference caused by such devices. Virtually every electronic device, intentionally or not, emits some form of electromagnetic radiation. While this condition could be tolerated when few devices existed, the increasing number of electronic devices has made the problem more acute. The problem has been exacerbated by the “improvement” in semiconductor devices, which allows them to operate at higher speeds, generally causing emission in the higher frequency bands where interference is more likely to occur.
Successful minimization of the interference problem, sometimes referred to as “electromagnetic compatibility” or “EMC”, generally requires that emissions from a given device be reduced by shielding and other means, and shielding be employed to reduce the sensitivity of a device to fields from other devices. Since shielding helps to reduce sensitivity to external fields as well as reduce emissions from the device, it is a common approach to a solution of the problem.
In the typical enclosure for a computer, for example, a number of electronic components are mounted therein. Such components include, for example, a central processing unit (CPU), a microprocessor, random access memory (RAM), read only memory (ROM) and other computer chips and electronic devices. Such electronic components may emit electromagnetic radiation during their operations. Data-processing devices, such as computers are typically manufactured with expansion slots to allow peripheral devices to be added to the computer or system. In such configurations, much of the electromagnetic radiation can leak out through the expansion slots to potentially interfere with other electronic components.
Typically, a number of cover plates are attached to a rear panel and block the expansion slots thereon. However, as the internal circuitry of computers has advanced, the cover plates can be inadequate to block the electromagnetic radiation. The inadequacy of the cover plates is believed to be due to gaps between the cover plates and the computer enclosure. In order to overcome such drawbacks, it is believed that a solution involves the implementation of a removable device containing EMC/RFI energy that can be contained by a continuous perimeter grounding seal or barrier. Such features are described in greater detail herein.